Our CPA works at a fixed cost which we agreed to after a lengthy discussion and initial upfront investigation fee for his time. We're really happy with this arrangement, and we use Xero (included with the CPA, not an extra) to make sure the work is being done and with no surprises.
We (SimpleLegal) just quietly rolled out an integration with Xero. Curious to know what you think of Xero.
I've been using it for years and never used a competing product except at a glance so I will be biased, but the clean interface makes me happy and for developers it's fantastic, being able to write integrations. Plus very simple to use after not much training, and Xero are serious about rolling out updates that are meaningful. That, for us is the best I think. Sure, still some ways to go as things like U.S. sales tax is tricky (and getting trickier) but at least having an interface like Xero let's you quickly see where you are at at any moment. Would recommend it to any size of business.